Anharmonic Vibrational Spectroscopy of Hydrogen-Bonded Systems Directly Computed from ab Initio Potential Surfaces: (H2O)n, n = 2, 3; Cl-(H2O)n, n = 1, 2; H+(H2O)n, n = 1, 2; H2O−CH3OH

Interaction energy and the shift in OH stretch frequency on hydrogen bonding for the H2O --> H2O, CH3OH --> H2O, and H2O --> CH3OH dimers

The ability to use calculated OH frequencies to assign experimentally observed peaks in hydrogen bonded systems hinges on the accuracy of the calculation. Here we test the ability of several commonly employed model chemistries--HF, MP2, and several density functionals paired with the 6-31+G(d) and 6-311++G(d,p) basis sets--to calculate the interaction energy (D(e)) and shift in OH stretch fundamental frequency on dimerization (delta(nu)) for the H(2)O --> H(2)O, CH(3)OH --> H(2)O, and H(2)O --> CH(3)OH dimers (where for X --> Y, X is the hydrogen bond donor and Y the acceptor). We quantify the error in D(e) and delta(nu) by comparison to experiment and high level calculation and, using a simple model, evaluate how error in D(e) propagates to delta(nu). We find that B3LYP and MPWB1K perform best of the density functional methods studied, that their accuracy in calculating delta(nu) is approximately 30-50 cm(-1) and that correcting for error in D(e) does little to heighten agreement between the calculated and experimental delta(nu). Accuracy of calculated delta(nu) is also shown to vary as a function of hydrogen bond donor: while the PBE and TPSS functionals perform best in the calculation of delta(nu) for the CH(3)OH --> H(2)O dimer their performance is relatively poor in describing H(2)O --> H(2)O and H(2)O --> CH(3)OH.

Surface organization of aqueous MgCl2 and application to atmospheric marine aerosol chemistry

Inorganic salts in marine aerosols play an active role in atmospheric chemistry, particularly in coastal urban regions. The study of the interactions of these ions with water molecules at the aqueous surface helps to elucidate the role of inorganic cations and anions in atmospheric processes. We present surface vibrational sum frequency generation (SFG) spectroscopic and molecular dynamics (MD) studies of aqueous MgCl(2) surfaces as models of marine aerosol. Spectroscopy results reveal that the disturbance of the hydrogen bonding environment of the air/aqueous interface is dependent on the MgCl(2) concentration. At low concentrations (< 1 M) minor changes are observed. At concentrations above 1 M the hydrogen bonding environment is highly perturbed. The 2.1 M intermediate concentration solution shows the largest SFG response relative to the other solutions including concentrations as high as 4.7 M. The enhancement of SFG signal observed for the 2.1 M solution is attributed to a larger SFG-active interfacial region and more strongly oriented water molecules relative to other concentrations. MD simulations reveal concentration dependent compression of stratified layers of ions and water orientation differences at higher concentrations. SFG and MD studies of the dangling OH of the surface water reveal that the topmost water layer is affected structurally at high concentrations (> 3.1 M). Finally, the MgCl(2) concentration effect on a fatty acid coated aqueous surface was investigated and SFG spectra reveal that deprotonation of the carboxylic acid of atmospherically relevant palmitic acid (PA) is accompanied by binding of the Mg(2+) to the PA headgroup.

Fundamental and overtone vibrational spectra of gas-phase pyruvic acid

Pyruvic acid (CH(3)COCOOH) is an important keto acid present in the atmosphere. In this study, the vibrational spectroscopy of gas-phase pyruvic acid has been investigated with special emphasis on the overtone transitions of the OH-stretch, with Delta v(OH) = 2, 4, 5. Assignments were made to fundamental and combination bands in the mid-IR. The two lowest energy rotational conformers of pyruvic acid are clearly observed in the spectrum. The lowest energy conformer possesses an intramolecular hydrogen bond, while the next lowest rotational conformer does not. This difference is clearly seen in the spectra of the OH vibrational overtone transitions, and it is reflected in the anharmonicities of the OH-stretching modes for each conformer. The spectra of the OH-stretching vibration for both conformers were investigated to establish the effect of the hydrogen bond on frequency, intensity, and line width.

Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes

In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

Anomalously strong effect of the ion sign on the thermochemistry of hydrogen bonded aqueous clusters of identical chemical composition

The sign preference of hydrogen bonded aqueous ionic clusters X±(H₂O)_i (n =1–5, X = F; Cl; Br) has been investigated using the Density Functional Theory and ab initio MP2 method. The present study indicates the anomalously large difference in formation free energies between cations and anions of identical chemical composition. The effect of vibrational anharmonicity on stepwise Gibbs free energy changes has been investigated, and possible uncertainties associated with the harmonic treatment of vibrational spectra have been discussed.

Dynamics of proton recombination with NO3- anion in water clusters

Recombination events of a proton with NO3- at (H2O)8 clusters are studied by molecular dynamics, using "on-the-fly" reliable ab initio MP2 potentials. The main findings are: (1) the lifetime of the ions is less than 1.2 picoseconds; (2) the recombination step invariably involves H3O+, not H5O2+; and (3) an essentially unique transition-state structure of H3O+/NO3- for recombination is found in all cases. Proton migration involves both H3O+ and H5O2+ species: Grotthuss and other mechanisms contribute.

Vibrational spectroscopy of (SO42−)∙(H2O)n clusters, n=1–5: Harmonic and anharmonic calculations and experiment

The vibrational spectroscopy of (SO4(2-)).(H2O)n is studied by theoretical calculations for n=1-5, and the results are compared with experiments for n=3-5. The calculations use both ab initio MP2 and DFT/B3LYP potential energy surfaces. Both harmonic and anharmonic calculations are reported, the latter with the CC-VSCF method. The main findings are the following: (1) With one exception (H2O bending mode), the anharmonicity of the observed transitions, all in the experimental window of 540-1850 cm(-1), is negligible. The computed anharmonic coupling suggests that intramolecular vibrational redistribution does not play any role for the observed linewidths. (2) Comparison with experiment at the harmonic level of computed fundamental frequencies indicates that MP2 is significantly more accurate than DFT/B3LYP for these systems. (3) Strong anharmonic effects are, however, calculated for numerous transitions of these systems, which are outside the present observation window. These include fundamentals as well as combination modes. (4) Combination modes for the n=1 and n=2 clusters are computed. Several relatively strong combination transitions are predicted. These show strong anharmonic effects. (5) An interesting effect of the zero point energy (ZPE) on structure is found for (SO4(2-)).(H2O)(5): The global minimum of the potential energy corresponds to a C(s) structure, but with incorporation of ZPE the lowest energy structure is C2v, in accordance with experiment. (6) No stable structures were found for (OH-).(HSO4-).(H2O)n, for n<or=5.

The calculated infrared spectrum of Cl-H2O using a new full dimensional ab initio potential surface and dipole moment surface

We report a full dimensional, ab initio-based global potential energy surface (PES) and dipole moment surface for Cl-H2O. Both surfaces are symmetric with respect to interchange of the H atoms. The PES is a fit to thousands of electronic energies calculated using the coupled-cluster method [CCSD(T)] with a moderately large basis (aug-cc-pVTZ). Vibrational energies and wave functions are accurately obtained using MULTIMODE. The wave function and dipole moment surface are used to calculate and analyze the pure infrared spectrum at 0 K which is compared with experiment. Vibrational energies and the infrared spectra for DOD and HOD/DOH are also presented.

Insertion of noble gas atoms into cyanoacetylene: an ab initio and matrix isolation study

A computational and experimental matrix isolation study of insertion of noble gas atoms into cyanoacetylene (HCCCN) is presented. Twelve novel noble gas insertion compounds are found to be kinetically stable at the MP2 level of theory, including four molecules with argon. The first group of the computationally studied molecules belongs to noble gas hydrides (HNgCCCN and HNgCCNC), and we found their stability for Ng = Ar, Kr, and Xe. The HNgCCCN compounds with Kr and Xe have similar stability to that of previously reported HKrCN and HXeCN. The HArCCCN molecule seems to have a weaker H-Ar bond than in the previously identified HArF molecule. The HNgCCNC molecules are less stable than the HNgCCCN isomers for all noble gas atoms. The second group of the computational insertion compounds, HCCNgCN and HCCNgNC, are of a different type, and they also are kinetically stable for Ng = Ar, Kr, and Xe. Our photolysis and annealing experiments with low-temperature cyanoacetylene/Ng (Ng = Ar, Kr, and Xe) matrixes evidence the formation of two noble gas hydrides for Ng = Kr and Xe, with the strongest IR absorption bands at 1492.1 and 1624.5 cm(-1), and two additional absorption modes for each species are found. The computational spectra of HKrCCCN and HXeCCCN fit most closely the experimental data, which is the basis for our assignment. The obtained species absorb at quite similar frequencies as the known HKrCN and HXeCN molecules, which is in agreement with the theoretical predictions. No strong candidates for an Ar compound are observed in the IR absorption spectra. As an important side product of this work, the data obtained in long-term decay of KrHKr+ cations suggest a tentative assignment for the CCCN radical.

Efficient configuration selection scheme for vibrational second-order perturbation theory

A fast algorithm of vibrational second-order Moller-Plesset perturbation theory is proposed, enabling a substantial reduction in the number of vibrational self-consistent-field (VSCF) configurations that need to be summed in the calculations. Important configurations are identified a priori by assuming that a reference VSCF wave function is approximated well by harmonic oscillator wave functions and that fifth- and higher-order anharmonicities are negligible. The proposed scheme has reduced the number of VSCF configurations by more than 100 times for formaldehyde, ethylene, and furazan with an error in computed frequencies being not more than a few cm(-1).

IR Spectrum of the H5O2+ Cation in the Context of Proton Disolvates L−H+−L

The H(5)O(2)(+) ion has been studied in chlorocarbon, benzene, and weakly coordinating anion environments to bridge the gap between the gas-phase and traditional condensed-phase investigations. Symmetrical cations of the type [H(5)O(2)(+) x 4Solv] are formed via H-bonding with the terminal O-H groups. In the infrared spectrum, the nu(s)OH and nu(as)OH vibrations behave in a manner similar to those of common water molecules: the stronger is the H-bonding interaction with the surroundings, the lower is the frequency shift. A consistent pattern of IR bands from the central O-H(+)-O group is identified, regardless of the strength of the interaction of H(5)O(2)(+) with its environment. Three intense bands develop: a (860-995 cm-1), b (1045-1101 cm(-1)), and c (1672-1700 cm(-1)), as well as two weak bands, d ( approximately 1300 cm(-1)) and e ( approximately 1400-1500 cm(-1)). These fingerprint bands are highly characteristic for vibrations of O-H-O group irrespective of formal charge. They are seen in symmetrical proton disolvates of the type L-H(+)-L, where L is an O-atom donor (alcohol, ether, ketone, phosphate, etc.), and in [A-H-A](-) acid salts (A(-) = oxyanion). The commonality is equivalency of the two O-atoms, a short O...O distance (ca. 2.40 Angstrom), and a flat-bottomed potential well for the bridging proton, that is, a short, strong, low-barrier H-bond. Assignments for bands a-e are suggested in an attempt to resolve inconsistencies between experimental and calculated data.

Jet cooled spectroscopy of H2DO+: Barrier heights and isotope-dependent tunneling dynamics from H3O+ to D3O+

The first high resolution spectroscopic data for jet cooled H2DO+ are reported, specifically via infrared laser direct absorption in the OH stretching region with a slit supersonic jet discharge source. Transitions sampling upper (0-) and lower (0+) tunneling states for both symmetric (nu1+ <-- 0+, nu1- <-- 0-, and nu1- <-- 0+) and antisymmetric (nu3+ <-- 0+ and nu3- <-- 0-) OH stretching bands are observed, where +/- refers to wave function reflection symmetry with respect to the planar umbrella mode transition state. The spectra can be well fitted to a Watson asymmetric top Hamiltonian, revealing band origins and rotational constants for benchmark comparison with high-level ab initio theory. Of particular importance are detection and assignment of the relatively weak band (nu1- <-- 0+) that crosses the inversion tunneling gap, which is optically forbidden in H3O+ or D3O+, but weakly allowed in H2DO+ by lowering of the tunneling transition state symmetry from D(3h) to C(2v). In conjunction with other H2DO+ bands, this permits determination of the tunneling splittings to within spectroscopic precision for each of the ground [40.518(10) cm(-1)], nu1 = 1 [32.666(6) cm(-1)], and nu3 = 1 [25.399(11) cm(-1)] states. A one-dimensional zero-point energy corrected potential along the tunneling coordinate is constructed from high-level ab initio CCSD(T) calculations (AVnZ, n = 3,4,5) and extrapolated to the complete basis set limit to extract tunneling splittings via a vibrationally adiabatic treatment. Perturbative scaling of the potential to match splittings for all four isotopomers permits an experimental estimate of DeltaV0 = 652.9(6) cm(-1) for the tunneling barrier, in good agreement with full six-dimensional ab initio results of Rajamaki, Miani, and Halonen (RMH) [J. Chem. Phys. 118, 10929 (2003)]. (DeltaV0 (RMH) = 650 cm(-1)). The 30%-50% decrease in tunneling splitting observed upon nu1 and nu3 vibrational excitations arises from an increase in OH stretch frequencies at the planar transition state, highlighting the transition between sp2 and sp3 hybridizations of the OHD bonds as a function of inversion bending angle.

Infrared studies of ionic clusters: The influence of Yuan T. Lee

Beginning in the mid-1980s, a number of innovative experimental studies on ionic clusters emerged from the laboratory of Yuan T. Lee combining infrared laser spectroscopy and tandem mass spectrometry. Coupled with modern electronic structure calculations, this research explored many facets of ionic clusters including solvation, structure, and dynamics. These efforts spawned a resurgence in gas-phase cluster spectroscopy. This paper will focus on the major areas of research initiated by the Lee group and how these studies stimulated and influenced others in what is currently a vibrant and growing field.

Ab Initio Study of the Isomeric Equilibrium of the HCN···H2O and H2O···HCN Hydrogen-Bonded Clusters

An ab initio study of the stability, spectroscopic properties, and isomeric equilibrium of the hydrogen-bonded HCN...H2O and H2O...HCN isomers is presented. Density functional theory and perturbative second-order MP2 and coupled-cluster CCSD(T) calculations were carried out and binding energies obtained with correlation-consistent basis sets including extrapolation to the infinity basis set level. At the best theoretical level, CCSD(T), the H2O...HCN complex is more stable than the HCN...H2O complex by ca. 6.3 kJ mol(-1). Rotational and vibrational spectra, including anharmonic corrections, are calculated. These calculated spectroscopic data are used to obtain thermochemical contributions to the thermodynamic functions and hence the Gibbs free energy. The relative free energies are used to estimate the equilibrium constant for isomerism. We find that under typical conditions of supersonic expansion experiments (T < 150 K) H2O...HCN is essentially the only isomer present. Furthermore, our calculations indicate that the hydrogen-bonded cluster becomes favorable over the separated moieties at temperatures below 200 K.

Infrared intensity in small ammonia and water clusters

Helium droplet technique has been used in order to measure the strength of the infrared absorption in small ammonia and water clusters as a function of size. Hydrogen bonding in ammonia and water dimers causes an enhancement of the intensity of the hydrogen stretching bands by a factor of four and three, respectively. Two types of the hydrogen bonded clusters show different size dependence of the infrared intensity per hydrogen bond. In ammonia (NH3)2 and (NH3)3 it is close to the crystal value. In water clusters, it increases monotonically with cluster size being in tetramers, a factor of two smaller than in the ice. The measured infrared intensity in water clusters is found to be a factor of two to three smaller as compared to the results of numerical calculations.

Prying Apart a Water Molecule with Anionic H-Bonding: A Comparative Spectroscopic Study of the X-·H2O (X = OH, O, F, Cl, and Br) Binary Complexes in the 600−3800 cm-1 Region

A detailed picture of the structural distortions suffered by a water molecule in direct contact with small inorganic anions (e.g., X = halide) is emerging from a series of recent vibrational spectroscopy studies of the gas-phase X-.H2O binary complexes. The extended spectral coverage (600-3800 cm(-1)) presently available with tabletop laser systems, when combined with versatile argon "messenger" techniques for acquiring action spectra of cold complexes, now provides a comprehensive survey of how the interaction evolves from an ion-solvent configuration into a three-center, two-electron covalent bond as the proton affinity of the anion increases. We focus on the behavior of H2O in the X-.H2O (X = Br, Cl, F, O, and OH) complexes, which all adopt asymmetric structures where one hydrogen atom is H-bonded to the ion while the other is free. The positions and intensities of the bands clearly reveal the mechanical consequences of both (zero-point) vibrationally averaged and infrared photoinduced excess charge delocalization mediated by intracluster proton transfer (X-.H2O --> HX.OH-). The fundamentals of the shared proton stretch become quite intense, for example, and exhibit extreme red-shifts as the intracluster proton-transfer process becomes available, first in the vibrationally excited states (F-.H2O) and then finally at the zero-point level (OH-.H2O). In the latter case, the loss of the water molecule's independent character is confirmed through the disappearance of the approximately 1600 cm(-1) HOH intramolecular bending transition and the dramatic (>3000 cm(-1)) red-shift of the shared proton stretch. An unexpected manifestation of vibrationally mediated charge transfer is also observed in the low frequency region, where the 2 <-- 0 overtones of the out-of-plane frustrated rotation of the water are remarkably intense in the Cl-.H2O and Br-.H2O spectra. This effect is traced to changes in the charge distribution along the X-.O axis as the shared proton is displaced perpendicular to it, reducing the charge transfer character of the H-bonding interaction and giving rise to a large quadratic contribution to the dipole moment component that is parallel to the bond axis. Thus, all of these systems are found to exhibit distinct spectral characteristics that can be directly traced to the crucial role of vibrationally mediated charge redistribution within the complex.

New Experimental and Theoretical Approach to the Heterogeneous Hydrolysis of NO2: Key Role of Molecular Nitric Acid and Its Complexes

Although heterogeneous chemistry on surfaces in the troposphere is known to be important, there are currently only a few techniques available for studying the nature of surface-adsorbed species as well as their chemistry and photochemistry under atmospheric conditions of 1 atm pressure and in the presence of water vapor. We report here a new laboratory approach using a combination of long path Fourier transform infrared spectroscopy (FTIR) and attenuated total reflectance (ATR) FTIR that allows the simultaneous observation and measurement of gases and surface species. Theory is used to identify the surface-adsorbed intermediates and products, and to estimate their relative concentrations. At intermediate relative humidities typical of the tropospheric boundary layer, the nitric acid formed during NO2 heterogeneous hydrolysis is shown to exist both as nitrate ions from the dissociation of nitric acid formed on the surface and as molecular nitric acid. In both cases, the ions and HNO3 are complexed to water molecules. Upon pumping, water is selectively removed, shifting the NO(3-)-HNO3(H2O)y equilibria toward more dehydrated forms of HNO3 and ultimately to nitric acid dimers. Irradiation of the nitric acid-water film using 300-400 nm radiation generates gaseous NO, while irradiation at 254 nm generates both NO and HONO, resulting in conversion of surface-adsorbed nitrogen oxides into photochemically active NO(x). These studies suggest that the assumption that deposition or formation of nitric acid provides a permanent removal mechanism from the atmosphere may not be correct. Furthermore, a potential role of surface-adsorbed nitric acid and other species formed during the heterogeneous hydrolysis of NO2 in the oxidation of organics on surfaces, and in the generation of gas-phase HONO on local to global scales, should be considered.

Theoretical Study of the Solvation of Fluorine and Chlorine Anions by Water

The solvation of fluoride and chloride anions (F(-) and Cl(-), respectively) by water has been studied using effective fragment potentials (EFPs) for the water molecules and ab initio quantum mechanics for the anions. In particular, the number of water molecules required to fully surround each anion has been investigated. Monte Carlo calculations have been used in an attempt to find the solvated system X(-)(H(2)O)(n) (X = F, Cl) with the lowest energy for each value of n. It is predicted that 18 water molecules are required to form a complete solvation shell around a Cl(-) anion, where "complete solvation" is interpreted as an ion that is completely surrounded by solvent molecules. Although fewer water molecules may fully solvate the Cl(-) anion, such structures are higher in energy than partially solvated molecules, up to n > or = 18. Calculations on the F(-) anion suggest that 15 water molecules are required for a complete solvation shell. The EFP predictions are in good agreement with the relative energies predicted by ab initio energy calculations at the EFP geometries.

Ab Initio Vibrational Calculations for H2SO4 and H2SO4·H2O: Spectroscopy and the Nature of the Anharmonic Couplings

Vibrational frequencies for fundamental, overtone, and combination excitations of sulfuric acid (H2SO4) and of sulfuric acid monohydrate cluster (H2SO4 x H2O) are computed directly from ab initio MP2/TZP potential surface points using the correlation-corrected vibrational self-consistent field (CC-VSCF) method, which includes anharmonic effects. The results are compared with experiment. The computed transitions show in nearly all cases good agreement with experimental data and consistent improvement over the harmonic approximation. The CC-VSCF improvements over the harmonic approximation are largest for the overtone and combination excitations and for the OH stretching fundamental. The agreement between the calculations and experiment also supports the validity of the MP2/TZP potential surfaces. Anharmonic coupling between different vibrational modes is found to significantly affect the vibrational frequencies. Analysis of the mean magnitude of the anharmonic coupling interactions between different pairs of normal modes is carried out. The results suggest possible mechanisms for the internal flow of vibrational energy in H2SO4 and H2SO4 x H2O.

Supersonically cooled hydronium ions in a slit-jet discharge: High-resolution infrared spectroscopy and tunneling dynamics of HD2O+

Jet-cooled high-resolution infrared spectra of partially deuterated hydronium ion (HD2O+) in the O-H stretch region (nu3 band) are obtained for the first time, exploiting the high ion densities, long absorption path lengths, and concentration modulation capabilities of the slit-jet discharge spectrometer. Least-squares analysis with a Watson asymmetric top Hamiltonian yields rovibrational constants and provides high level tests of ab initio molecular structure predictions. Transitions out of both the lower (nu3(+)<--0(+)) and the upper (nu3(-)<--0(-)) tunneling levels, as well as transitions across the tunneling gap (nu3(-)<--0(+)) are observed. The nu3(-)<--0(+) transitions in HD2O+ acquire oscillator strength by loss of D(3h) symmetry, and permit both ground-state-[27.0318(72) cm(-1)] and excited-state-[17.7612(54) cm(-1)]-tunneling splittings to be determined to spectroscopic precision from a single rovibrational band. The splittings and band origins calculated with recent high level ab initio six-dimensional potential surface predictions for H3O+ and isotopomers [X. C. Huang, S. Carter, and J. M. Bowman, J. Chem. Phys. 118, 5431 (2003); T. Rajamaki, A. Miani, and L. Halonen, J. Chem. Phys. 118, 10929 (2003)] are in very good agreement with the current experimental results.

Ab initio potential energy and dipole moment surfaces for H5O2+

Full-dimensional ab initio potential energy surface (PES) and dipole moment surface (DMS) are reported for H(5)O(2) (+). Tens of thousands of coupled-cluster [CCSD(T)] and second-order Moller-Plesset (MP2) calculations of electronic energies, using aug-cc-pVTZ basis, were done. The energies were fit very precisely in terms of all the internuclear distances, using standard least-square procedures, however, with a fitting basis that satisfies permutational symmetry with respect to like atoms. The H(5)O(2) (+) PES is a fit to 48 189 CCSD(T) energies, containing 7962 polynomial coefficients. The PES has a rms fitting error of 34.9 cm(-1) for the entire data set up to 110 000 cm(-1). This surface can describe various internal floppy motions, including the H atom exchanges, monomer inversions, and monomer torsions. First- and higher-order saddle points have been located on the surface and compared with available previous theoretical work. In addition, the PES dissociates correctly (and symmetrically) to H(2)O+H(3)O(+), with D(e)=11 923.8 cm(-1). Geometrical and vibrational properties of the monomer fragments are presented. The corresponding global DMS fit (MP2 based) involves 3844 polynomial coefficients and also dissociates correctly.

Predissociation spectroscopy of the argon-solvated H5O2+ “zundel” cation in the 1000–1900 cm−1 region

Predissociation spectra of the H5O2+.Ar(1,2) cluster ions are reported in the 1000-1900 cm(-1) region. The weakly bound argon atoms enable investigation of the complex in a linear action mode, and the resulting spectra are much simpler than those reported previously in this region [Asmis et al., Science 299, 1375 (2003) and Fridgen et al., J. Phys. Chem. A 108, 9008 (2004)], which were obtained using infrared multiphoton dissociation of the bare complex. The observed spectrum consists of two relatively narrow bands at 1080 and 1770 cm(-1) that are likely due to excitation of the shared proton and intramolecular bending vibrations of the two water molecules, respectively. The narrow linewidths and relatively small (60 cm(-1)) perturbation introduced by the addition of a second argon atom indicate that the basic "zundel" character of the H5O2+ ion survives upon complexation.

Matrix isolation infrared and ab initio study of the hydrogen bonding between formic acid and water

The infrared spectra of the formic acid-water complexes isolated in argon matrices are reported. Both supersonic jet expansion and a conventional effusive source followed by trapping in solid argon at 10K are used to obtain the matrices. The experimental IR spectra are compared to the data obtained from high level ab initio (MP2) and DFT (B3LYP) calculations with 6-311++G(d,p) and aug-cc-pVTZ basis sets. The complex formation results in red shifts in the C=O and O-H stretching vibrations and a blue shift in the C-O stretching vibration of formic acid. The O-H stretching modes of water also exhibit pronounced red shifts. Both the MP2 and B3LYP calculations located three minima corresponding to cyclic HCOOH...H2O complexes with two hydrogen bond interactions. The binding energies are -10.3, -5.1, and -3.5 kcal mol(-1), respectively, for the three complexes at the MP2/ aug-cc-pVTZ level, corrected for the basis set superposition error (BSSE) using the Boys-Bernardi counterpoise scheme. Comparison of the calculated frequencies of the three complexes with the matrix IR spectrum reveals that the lowest energy complex is formed. In addition, a complex of formic acid with two water molecules is observed.

Formation of novel rare-gas molecules in low-temperature matrices

Progress in the study of a new class of chemically bound compounds of noble-gas atoms is reviewed. The focus is on rare-gas molecules of the form HNgY, where Ng is a noble-gas atom and Y is an electronegative group, prepared by photolysis of HY in the rare-gas matrix. Other related types of new molecules of noble-gas atoms are discussed as well. Topics discussed in this review include: (a) The nature of bonding and the energetic stability of the compounds. (b) The vibrational spectroscopy of the molecules, and its role in identification of the species. (c) The mechanism and dynamics of photochemical formation of HNgY in the matrix, and the pathways for thermal and infrared (IR)-induced decomposition. Specifically, attention is given to the issue of "direct" formation following photolysis of HY versus "delayed" formation involving H atom diffusion. (d) Molecules of the lighter rare gases Ar, Ne, and He, focusing on the experimentally prepared HArF and on theoretical predictions suggesting the existence of other molecules. (e) The most-recently discovered photochemically induced insertion compounds of Ng into hydrocarbons, such as HXeCCH. (f) Clusters of HNgY with other molecules. The possible existence of neat aggregates and crystals of HNgY. The reviewed state-of-the-art suggests this field is at an early stage of development with major open questions bearing on the surprising properties of the molecules and on the formation mechanisms. These are part of the challenge for the future.

Ab initiovibrational state calculations with a quartic force field: Applications to H2CO, C2H4, CH3OH, CH3CCH, and C6H6

For polyatomic molecules, n-mode coupling representations of the quartic force field (nMR-QFF) are presented, which include terms up to n normal coordinate couplings in a fourth-order polynomial potential energy function. The computational scheme to evaluate third-and fourth-order derivatives by finite differentiations of the energy is fully described. The code to generate the nMR-QFF has been implemented into GAMESS program package and interfaced with the vibrational self-consistent field (VSCF) and correlation corrected VSCF (cc-VSCF) methods. As a demonstration, fundamental frequencies have been calculated by the cc-VSCF method based on 2MR-QFF for formaldehyde, ethylene, methanol, propyne, and benzene. The applications show that 2MR-QFF is a highly accurate potential energy function, with errors of 1.0-1.9% relative to the experimental value in fundamental frequencies. This approach will help quantitative evaluations of vibrational energies of a general molecule with a reasonable computational cost.